Thread overview
Block statements and memory management
Mar 16
Murilo
Mar 16
Meta
Mar 16
Dennis
Mar 16
spir
Apr 06
Murilo
March 16
Does anyone know if when I create a variable inside a scope as in
{int a = 10;}
it disappears complete from the memory when the scope finishes? Or does it remain in some part of the memory? I am thinking of using scopes to make optimized programs that consume less memory.
March 16
On Saturday, 16 March 2019 at 03:47:43 UTC, Murilo wrote:
> Does anyone know if when I create a variable inside a scope as in
> {int a = 10;}
> it disappears complete from the memory when the scope finishes? Or does it remain in some part of the memory? I am thinking of using scopes to make optimized programs that consume less memory.

It depends on how the compiler translates and optimizes the code. An integer variable like `a` in your example might never exist in memory at all, if the compiler can allocate a register for it until it goes out of scope. The easiest way to find out is to look at a disassembly of the compiled code.
March 16
On Saturday, 16 March 2019 at 03:47:43 UTC, Murilo wrote:
> Does anyone know if when I create a variable inside a scope as in
> {int a = 10;}
> it disappears complete from the memory when the scope finishes? Or does it remain in some part of the memory? I am thinking of using scopes to make optimized programs that consume less memory.

I'd recommend against these sorts of micro-optimizations. Compilers are every good at doing this kind of thing manually so you don't have to worry about it and can concentrate on the actual logic of your program.
March 16
On Saturday, 16 March 2019 at 03:47:43 UTC, Murilo wrote:
> Does anyone know if when I create a variable inside a scope as in
> {int a = 10;}
> it disappears complete from the memory when the scope finishes? Or does it remain in some part of the memory? I am thinking of using scopes to make optimized programs that consume less memory.

In general, you want variables to have no larger scope than needed, so in large functions reducing the scope may be useful. When it comes to efficiency however, doing that is neither necessary nor sufficient for the compiler to re-use registers / stack space. I looked at the assembly output of DMD for this:
```
void func(int a);

void main()
{
    {
    int a = 2;
    func(a);
    }
    {
    int b = 3;
    func(b);
    }
}
```
Without optimizations (the -O flag), it stores a and b on different places in the stack.
With optimizations, the values of a and b (2 and 3) are simply loaded in the EDI register before the call.
Removing the braces doesn't change anything about that.
The compiler does live variable analysis [1] as well as data-flow analysis [2] to figure out that it's only needed to load the values 2 and 3 just before the function call. This is just a trivial example, but the same applies to larger functions.

In any case, for better memory efficiency I'd consider looking at reducing dynamic allocations such as new or malloc. Memory on the stack is basically free compared to that, so even if adding lots of braces to your code reduces stack memory, chances are it's a low leverage point.

[1] https://en.wikipedia.org/wiki/Live_variable_analysis
[2] https://en.wikipedia.org/wiki/Data-flow_analysis
March 16
On 16/03/2019 11:19, Dennis via Digitalmars-d-learn wrote:
> On Saturday, 16 March 2019 at 03:47:43 UTC, Murilo wrote:
>> Does anyone know if when I create a variable inside a scope as in
>> {int a = 10;}
>> it disappears complete from the memory when the scope finishes? Or does it remain in some part of the memory? I am thinking of using scopes to make optimized programs that consume less memory.
> 
> In general, you want variables to have no larger scope than needed, so in large functions reducing the scope may be useful. When it comes to efficiency however, doing that is neither necessary nor sufficient for the compiler to re-use registers / stack space. I looked at the assembly output of DMD for this:
> ```
> void func(int a);
> 
> void main()
> {
>      {
>      int a = 2;
>      func(a);
>      }
>      {
>      int b = 3;
>      func(b);
>      }
> }
> ```
> Without optimizations (the -O flag), it stores a and b on different places in the stack.
> With optimizations, the values of a and b (2 and 3) are simply loaded in the EDI register before the call.
> Removing the braces doesn't change anything about that.
> The compiler does live variable analysis [1] as well as data-flow analysis [2] to figure out that it's only needed to load the values 2 and 3 just before the function call. This is just a trivial example, but the same applies to larger functions.
> 
> In any case, for better memory efficiency I'd consider looking at reducing dynamic allocations such as new or malloc. Memory on the stack is basically free compared to that, so even if adding lots of braces to your code reduces stack memory, chances are it's a low leverage point.
> 
> [1] https://en.wikipedia.org/wiki/Live_variable_analysis
> [2] https://en.wikipedia.org/wiki/Data-flow_analysis

Just to add a bit on what has been said:
* Register allocation (see wikipedia) is a well-researched area.
* By coding that way, you force the compiler to optimise *a certain way* which may prevent it to perform other, more relevant optimisations.
* You cannot beat the knowledge in that domain, it is simply too big and complex, just be confident.
diniz

March 16
On Sat, Mar 16, 2019 at 01:21:02PM +0100, spir via Digitalmars-d-learn wrote:
> On 16/03/2019 11:19, Dennis via Digitalmars-d-learn wrote:
[...]
> > In any case, for better memory efficiency I'd consider looking at reducing dynamic allocations such as new or malloc. Memory on the stack is basically free compared to that, so even if adding lots of braces to your code reduces stack memory, chances are it's a low leverage point.
> > 
> > [1] https://en.wikipedia.org/wiki/Live_variable_analysis [2] https://en.wikipedia.org/wiki/Data-flow_analysis
> 
> Just to add a bit on what has been said:
> * Register allocation (see wikipedia) is a well-researched area.
> * By coding that way, you force the compiler to optimise *a certain way*
> which may prevent it to perform other, more relevant optimisations.
> * You cannot beat the knowledge in that domain, it is simply too big
> and complex, just be confident.
[...]

And to add even more to that: before embarking on micro-optimizations of this sort, always check with a profiler whether or not the bottleneck is even in that part of the code. Often I find myself very surprised at where the real bottleneck is, which is often nowhere near where I thought it should be.  Also, check with a memory profiler to find out where the real heavy memory usage points are.  It may not be where you thought it was.

Generally speaking, in this day and age of highly-optimizing compilers, premature optimization is the root of all evils, because it uglifies your code and makes it hard to maintain for little or no gain, and sometimes for *negative* gain, because by writing code in an unusual way, you confuse the optimizer as to your real intent, thereby reducing its effectiveness at producing optimized code. Don't optimize until you have verified with a profiler where your bottlenecks are. It takes a lot of time and effort to write code this way, so make it count by applying it where it actually matters.

Of course, this assumes you use a compiler with a powerful-enough optimizer.  I recommend ldc/gdc if performance is important to you. Dmd compiles somewhat faster, but at the cost of poorer codegen.


T

-- 
The early bird gets the worm. Moral: ewww...
March 16
On Saturday, 16 March 2019 at 03:47:43 UTC, Murilo wrote:
> Does anyone know if when I create a variable inside a scope as in
> {int a = 10;}
> it disappears complete from the memory when the scope finishes? Or does it remain in some part of the memory? I am thinking of using scopes to make optimized programs that consume less memory.

Others have made good points in this thread, but what is missing is that indeed scopes _can_ be used beneficially to reduce memory footprint.

I recommend playing with this code on d.godbolt.org:
```
void func(ref int[10] a); // important detail: pointer

void foo()
{
    {
        int[10] a;
        func(a);
    } {
        int[10] b;
        func(b);
    }
}
```

Because the variable is passed by reference (pointer), the optimizer cannot merge the storage space of `a` and `b` _unless_ scope information is taken into account. Without taking scope into account, the first `func` call could store the pointer to `a` somewhere for later use in the second `func` call for example. However, because of scope, using `a` after its scope has ended is UB, and thus variables `a` and `b` can be used.

GDC uses scope information for variable lifetime optimization, but LDC and DMD both do not.
For anyone interested in working on compilers: adding variable scope lifetime to LDC (not impossibly hard) would be a nice project and be very valuable.

-Johan



April 06
On Saturday, 16 March 2019 at 15:53:26 UTC, Johan Engelen wrote:
> On Saturday, 16 March 2019 at 03:47:43 UTC, Murilo wrote:
>> Does anyone know if when I create a variable inside a scope as in
>> {int a = 10;}
>> it disappears complete from the memory when the scope finishes? Or does it remain in some part of the memory? I am thinking of using scopes to make optimized programs that consume less memory.
>
> Others have made good points in this thread, but what is missing is that indeed scopes _can_ be used beneficially to reduce memory footprint.
> -Johan

I would like to thank everyone for your help, those informations were very helpful.